1. Field of the Invention
The present invention relates to a diversity radio receiver and, more particularly, to a reception local oscillator offset diversity radio receiver in a digital radio communication system.
2. Description of the Related Art
In digital radio communication and particularly mobile communication, it is not easy to always assure communication quality due to the presence of fading. For this reason, diversity techniques have been frequently used.
For example, a plurality of antennas spaced apart from each other so as to decrease the spatial correlation of the reception levels are used to perform reception by switching the reception antenna to an antenna having a higher level, or space diversity for synchronizing the frequencies and phases of input RF signals, or IF frequencies. Alternatively, offset transmission diversity for modulating carrier frequencies having a predetermined difference from two or more transmission base stations with the same baseband signal, simultaneously transmitting the modulated signals, and causing one antenna at the reception unit to receive the modulated signals.
In the former diversity scheme, since a circuit for comparing the two reception levels and a circuit for selecting a signal on the basis of the comparison result are required, or a circuit for detecting the difference of frequencies and phases between input signals and a circuit for adjusting them to be null, the circuit arrangement is undesirably complicated.
On the other hand, in the latter diversity scheme, the predetermined frequency difference between the plurality of base stations spaced apart from each other must be assured, and a circuit for minimizing a time lag between the modulation baseband signals of the respective stations is required. In addition, this scheme requires an implementation of circuit arrangements in both the transmission and reception units. Two or more electric waves are required in principle, thereby wasting the electric waves.